TELEPHONE No. 77

Telephone used in Automatic exchange areas served by
Siemens systems..

It was introduced when the first SB&Co. exchanges were
brought into service, which was in 1918 (Grimsby), Stockport (1919) and
Southampton (1923). It was also used on the Siemens RAX at Ramsey (Hunts.)
in 1921 as well and an exchange called Hurley.

There were no SB&Co. public exchanges in Britain before the Great War. The Siemens Brothers works in Woolwich had an auto PABX but this almost
certainly used German Siemens telephones with monster 'half moon'-type dials.

Mk 234 differed from Mk 235 as the Mk 234 had the terminals R and TR omitted.

Made by Siemens Brothers and thought to be introduced around 1916 as a
Siemens of that date depicts the set.

This telephone had a high failure rate when in service and this is
generally attributed to the capacitor wire being terminated at the junction
of the dial off normal spring and the impulse spring. A high
resistance fault on the impulse springs would result in failure of the bell
circuit. The Telephone No. 105 is identical to
the No. 77 except that the capacitor wire has been repositioned.